1. The Field of the Invention
This invention relates generally to the conversion of plasticized synthetic plastic parisons into hollow shaped articles by blow-molding.
More particularly, the invention relates to a method of effecting such a conversion by blow-molding in accordance with the so-called "biaxial stretching" technique.
In particular, the invention relates to a method of the just-described kind wherein parisons are first converted into hollow blanks, and the blanks are thermally tempered in two or more tempering stages before they are converted into the final shaped article.
2. The State of the Art
It is known from German allowed application No. 1,479,449 to make shaped hollow articles from synthetic thermoplastic material in a two-stage blow-molding operation. A plasticized tubular parison is extruded, usually continuously, from an extrusion machine and successive sections of the extruded parisons are confined in an open-and-shut mold. Either separately, or by the action of the closing mold, one end of the parison section confined in the mold is squeezed shut. The mold with the confined parison section is located at a blowing station where a blowing mandrel is inserted into the confined parison section and injects pressurized gas into the same, thereby pneumatically expanding the parison section into conformance with the size and shape of the mold cavity and producing the desired hollow shaped article.
The parison can be transferred from the extrusion machine to the blowing station in a variety of ways. For example, the mold may shuttle back and forth between the extrusion machine where it receives the parison sections and the blowing station where the pneumatic conversion takes place. However, it is also possible to use tongs or similar instrumentalities which engage a parison section that is severed from the parison being extruded, and transfer this section to the blowing station for confinement in the mold which, in such a case, does not move away from the blowing station. Similarly, there are ways of effecting the penumatic expansion of the parison section without resorting to the use of a blowing mandrel. For instance, a blowing needle may be laterally inserted through the wall of the parison section to expand the latter.
For reasons known to those skilled in the art, this type of blow-molding is not satisfactory in all circumstances. Therefore, in order to provided needed improvements the so-called "biaxial stretching" method of blow-molding was developed, which is disclosed, e.g. in German published application No. 2,161,066.
The initial operations in the biaxial stretching method are similar to those in the "simple" blow-molding method described above. A parison is extruded and parison sections confined in an open-and-shut mold. However, in the biaxial stretching method the shape of the mold cavity is not identical with the shape desired for the final article; it corresponds thereto only approximately. The same is true of the mold-cavity dimensions which are larger than those of the parison but smaller than the dimensions desired for the final article. Since this mold thus does not produce the final article--as in the "simple" blow-molding method--it is known as a "premold".
The parison section is expanded to the shape of the premold cavity, in the manner described earlier, thereby producing a hollow blank. The open-and-shut premold is then opened and the hollow blank is transferred into a final mold wherein the blank is again pneumatically expanded; since the cavity of the final mold has the shape and dimensions desired for the final article, this pneumatic expansion of the hollow blank accordingly produces the final article.
Transfer of the blank to the final mold can be effected in various ways. The final mold can shuttle from the final blowing station to the first blowing station, close about the hollow blank after the premold has exposed the same, and then travel with the enclosed blank to the final blowing station. Alternatively, the final mold may remain at the final blowing station, and the blank may be tranferred from the first blowing station to the final mold, for example by being suspended from a blowing mandrel which has already been used at the first blowing station to expand the parison section into the shape of the blank, and which blowing mandrel may be movable between the first and final blowing stations. However, the aforementioned blowing mandrel may instead remain at the first blowing station and the blank transfer may be effected by means of grippers or tongs. If the blowing mandrel used in conjunction with the premold is of the type which transfers the blank from the premold to the final mold, it may also be used to expand the blank in the final mold to the shape of the final article; alternately, a separate blowing mandrel may be provided for this purpose.
The reasons for the development of the biaxial-stretching type of blow-molding are known to those skilled in the art. They included, in particular, the assumption that the hollow blank would cool down during its transfer from the first blowing station to the final blowing station, so that the molecular stretching effect which results during axial and radial expansion of the blank in the final mold, would be "fixed", i.e. made irreversible, due to the lowered temperature of the blank material. This is desirble because, if attainable, it will significantly improve the strength of the finished hollow article and make it possible to either reduce the wall thickness of the article without loss of strength, or to obtain increased strength from the same wall thickness as was used previously.
Once placed in practical use, however, the biaxial-stretching method of blow-molding was found not to justify all the expectations which had been placed in it. Although the theory of molecular stretching or orienting, and the strength-improvements to be gained from "fixing" the molecules in their stretched condition, is correct, it was found that in practice it is extremely difficult to get the blanks to just the temperature which they are required to have prior to the final blowing if the required advantages are to accrue.
In particular, it was determined that as a rule the transporting time from the premold to the final mold is insufficient for the blanks to cool down to the desired lower temperature. To overcome this difficulty a method of blow-molding with biaxial stretching was developed which resorts to so-called "intermediate tempering" or conditioning of the blank.
This method operates in the same way as the above-described conventional biaxial stretching blow-molding method--until the point at which the blank leaves the premold. Rather than moving from the premold directly into the final mold, the blank now moves to a tempering station at which it undergoes thermal tempering, i.e. thermal conditioning. As disclosed in German published application No. 23 43 125, the tempering station may have a chamber into which the blank is introduced, whereupon air is blown through the chamber in order to adjust the temperature of the blank to a level at which it has the optimum temperature for the subsequent biaxial stretching in the final mold. In addition to the positive influence of this air, the dwell time of the blank in the chamber can also be selected in such a manner that a temperature equalization takes place over the active cross-section of the wall or walls of the blank, i.e. so that the temperature at the core of the wall is not or not substantially different from the temperature of the exposed outer surfaces of the wall.
Instead of using a chamber it is also possible to employ a mold which closes about the blank and cools the same by heat-exchange therewith. Such molds can be cooled by means of water or other cooling fluids.
Another possibility is to simply expose the blank at the tempering station and direct cooling air into contact with it, or even to let the blank dwell at the tempering station in exposed condition until the desired temperature is reached.
This "intermediate tempering" method represents a definite improvement over the prior state of the art. However, it is still not really satisfactory, basically for two reasons. Firstly, it decreases the working speed of machines which employ this method, because for the time period during which a blank is located at the tempering station, the just-produced next-following blank cannot be moved from the preform to the tempering station. Conversely of course, the final mold from which the preceding final article has been ejected, must remain idle and wait until the blank at the tempering station is ready to be passed on to the final mold. Secondly, it is not possible to adjust the blank to a precise temperature in the instances where tempering of the blank to an approximate temperature is not adequate or not acceptable.